The invention relates to a method and arrangement for embedding supplemental data in an information signal, comprising the step of embedding supplemental data samples at predetermined positions of said information signal. The invention also relates to a method and arrangement for extracting the supplemental data from such an information signal.
There is a growing need to accommodate watermarks in audio and video signals. Watermarks are supplemental data messages embedded in multimedia assets, preferably in a perceptually invisible manner. They comprise information, for example, about the source or copyright status of documents and audiovisual programs. They can be used to provide legal proof of the copyright owner, and allow tracing of piracy and support the protection of intellectual property.
A known method of embedding supplemental data in an information signal as defined in the opening paragraph is disclosed, inter alia, in International Patent Application WO-A-98/33324. In this prior-art method, a watermark pattern is embedded in a (sigma-)delta-modulated audio signal. The watermark is embedded in the encoded audio signal by modifying selected bits thereof. For example, every 100th bit is replaced by a bit of the watermark pattern. The step of modifying the encoded audio signal is carried out inside the feedback loop of the encoder so as to compensate the effect of the modification in subsequent encoding steps.
The prior-art method is envisaged for recording high-quality audio on the audio version of the Digital Versatile Disk (DVD). A sampling frequency of 2,822,400 Hz (64*44,100) will be used to yield a signal-to-noise ratio of 115 dB. Replacing every 100th bit of the sigma-delta-modulated audio signal by a watermark bit at the expense of only 1 dB increases the quantization noise. This corresponds to watermark bit rate of about 28000 bits per second.
The above-mentioned patent application WO-A-98/33324 also discloses an arrangement for extracting the watermark. A synchronization bit pattern (hereinafter sync pattern for short) is accommodated in the bit stream to identify the supplemental data bit positions. The arrangement comprises a divider stage and a sync detector. The divider stage divides the bit rate by the number of bits by which the watermark bits are spaced apart (e.g. 100 if every 100th bit of the signal is a supplemental data bit). The sync detector changes the phase of the divider stage until the sync pattern is found. Such a sync detector includes a relatively long shift register or serial-to-parallel converter to store a portion of the bit stream. If every Mth bit of the signal is a supplemental data bit and the sync pattern comprises N bits, the sync detector must necessarily store (Nxe2x88x921)xc2x7M+1 bits.
German Patent Application DE-A-37 17 315 discloses such a known sync detector in more detail. In this publication, every 15th bit of a signal is a supplemental bit and the sync pattern is a 4-bit word. In accordance therewith, the shift register (serial-to-parallel converter 5 in FIG. 2 of DE-A-37 17 315) holds 46 bits.
In order to reduce the length of the shift register, it is proposed in Applicant""s non-published co-pending patent application PHN 17.148 to make the spacing between the sync pattern bits considerably smaller than the spacing between the watermark bits. However, this solution affects the coding performance and increases the signal-to-noise ratio of the sigma-delta modulator.
It is an object of the invention to provide a method of embedding supplemental data in an information signal, which allows the supplemental data bit positions to be detected in an alternative manner.
To this end, the method in accordance with the invention is characterized in that the information signal comprises at least two signal channels, the method including the step of embedding the same supplemental data samples at corresponding positions of said channels. The transmission of a sync pattern can now be omitted because the position of the embedded watermark bits can easily be found by searching the positions in which the at least two channel samples are identical.
The corresponding method of extracting the supplemental data from the information signal comprises the steps of comparing a first sequence of predetermined first channel signal samples with a corresponding second sequence of second channel signal samples, displacing said first and second sequence by one sample position, and repeating said comparison step for said displaced first and second sequence as long as the sequences are not identical.